Giant Enhancement of Optical Nonlinearity by Manipulating Guest Molecular Stacking Modes in Metal–Organic Frameworks

The influence of guest stacking interactions in host-guest (H-G) MOF composites on third-order nonlinear optical (NLO) performance remains largely unknown. Herein, we propose for the first time a noncovalent aggregate confinement strategy for synthesizing H-G MOF composites with different guest stacking modes. And [perylene₂]_<i>n</i> (α-Pe) and [perylene]_<i>n</i> (β-Pe) were selected as guests and confined into a novel Ca-based MOF {[Ca(TBAPy)(DMA)₂]·3DMA·[N(CH₃)₂]·H₂O}_<i>n</i> (Ca-MOF-pts). The NLO results showed that compared to β-Pe@Ca-MOF-pts, the saturable absorption (SA) and self-defocusing properties of α-Pe@Ca-MOF-pts were increased by 2.71-fold and 3.82-fold, respectively. Interestingly, α/β-Pe@Ca-MOF-pts can be transformed into α/β-Pe@Ca-MOF-flu (Ca-MOF-flu = {[Ca_1.5(TBAPy)(H₂O)₂]·DMA·[N(CH₃)₂]·2H₂O}_<i>n</i>) through self-adaptive topological evolution, and the corresponding NLO absorption signal change from SA to reverse saturable absorption (RSA). As expected, compared to β-Pe@Ca-MOF-flu, the RSA and self-defocusing properties of α-Pe@Ca-MOF-flu are improved by 2.94-fold and 4.07-fold, respectively, demonstrating the importance of guest stacking modes. Theoretical calculation and transient absorption spectra indicated the enhancement of NLO performance was attributed to the large π-π overlap of α-Pe, which promoted the electron delocalization/transfer and optimized the cross-sectional of the ground state and excited state. This study provides a new strategy for developing H-G MOF composites with excellent NLO properties.